An actuator of the aforedescribed kind may be used to execute regulating movements between three or more different and distinct operating positions. Thus, when regulating a valve it is often desired to adjust the valve reliably in both its half-open position, corresponding to a 90.degree. rotation of the valve spindle, and a fully-open position, corresponding to 180.degree. rotation of said spindle. Many hitherto known actuators are not able to satisfy this requirement, since they will only permit the valve to be regulated between a zero or closed position and an end position corresponding to a position in which the valve is fully open or in which the valve spindle has been rotated through 180.degree.. U.S. Pat. No. 4,087,074 (Massey et al) describes an example of a known valve of this kind, which has a return spring coupled to an independent fluid actuated piston to assist in spring compression on the forward stroke.
German Pat. No. 2,405,297 (Kurten) describes an actuator having four pistons in symmetrical arrangements, which is a rather complicated and expensive design.
German Pat. No. 2,508,683 (Yazaki) describes an actuator having, like Massey et al, a spring actuated additional piston, also providing a manually operated wheel for adjusting the end positions.
Consequently, when wishing to make intermediate adjustments, it has either been necessary to make such adjustments manually or to employ other, more complicated solutions, in which mutually different actuators are placed one upon the other, or the like. In certain cases it has been necessary to ignore fluid actuators of the kind described and to employ totally different solutions, for example solutions which require the use of electricity and which in practice greatly increase the costs entailed.